IV. j INITIAL STRUCTURE OF THE GERM 267 



be made to the relation established by Boveri. as a result of the 

 examination of sea-urchin larvae (Echinus), between the number 

 of chromosomes on the one hand and the size of the nucleus 

 and the size of the cell on the other. 



The number of chromosomes in a larva, or part of a larva, may 

 be varied in the following ways. 



1. In merogony the number is half the normal number, i.e. 

 n. (The larva is termed by Boveri hemikaryotic arrhenokaryotic.) 



2. In artificially parthenogenetic larvae it is also n. (Hemi- 

 karyotic thelykaryotic.) 



3. In ' monaster ' eggs the number is twice the normal, i. e. 

 4 n. (Diplokaryotic.) 



This condition is brought about by shaking the eggs, and so 

 preventing the division of the centrosome. The pronuclei unite 

 and enter a resting stage. From this the chromosomes emerge 

 in the full number and split, but since the centrosome remains 

 single, there is no cell-division, and the chromosomes once 

 more form a resting nucleus, but now in twice the normal 

 number. 



Later the centrosome divides and the egg develops normally. 



4. If the eggs are kept for twenty-four hours in sea-water and 

 then fertilized with sperm which has been weakened in dilute 

 potash the male pronucleus lags behind its centre. The latter 

 divides about the female pronucleus, and thus in the two-celled 

 stage one blastomere has a male and a female nucleus, and there- 

 fore 2 n chromosomes (amphikaryotic), while the other has only 

 a female (thelykaryotic). In this stage, or subsequently, the 

 male unites with the female pronucleus, and so one-half, or one- 

 quarter, possibly only one-eighth, of the larva has nuclei with 

 2 n, the remainder nuclei with n chromosomes. 



5. It sometimes happens in dispermy that the two pairs of 

 centrosomes remain apart without forming a quadripolar figure 

 (Fig. 162). Such eggs, as we have seen, develop normally. 

 Sooner or later the cells become divided into two sets, one with 

 nuclei containing 2 n chromosomes (amphikaryotic), derived from 

 the female and one of the male pronuclei, the other having n 

 chromosomes from the remaining male (arrhenokaryotic). 



A superficial examination shows that the nuclei of hemikaryotic 

 embryos and larvae are smaller but more numerous than in 



